With tubular belt conveyors, also referred to as hose-type belt conveyors or pipe conveyors, mechanical means are used to form the conveyor belt into a closed tube or pipe in the conveying zone proper, downstream of the loading area and up-stream of the discharge area. In the case of the more recent structures the closing of the "pipe" is accomplished by overlapping of the conveyor belt alongs both its edges. To permit the formation of the tubular belt, the conveyor belt structure must conform to high requirements not comparable with those of a normal conveyor belt. While requirements, such as good trough formation, high tensile strength, corrosion protection of the steel reinforcement, and resistance of the belt running and supporting sides are in the foreground with normal conveyor belts, it is additionally important with tubular belt conveyors to achieve the best possible formation of a circle with a tight overlap that does not protrude. To accomplish that, the external marginal zone of the conveyor belt must not tip outwardly and the internal marginal zone must not tip inwardly too much. Moreover, in spite of the pipe formation, the conveyor belt must remain well deflectable both in the vertical and horizontal planes, in other words in curves. This means that it must remain stable in shape and true to track no matter what the bending. Finally, no twisting should occur in the conveyor belt formed into a pipe in the sense that the overlap usually located at the top would shift more or less to one or the other side.
Usually the structure including embedded steel ropes as tension barriers is preferred if a conveyor belt has to accommodate high tensile forces either because of the great length of the conveying distance or due to high loading. In this context the small elastic elongation of the steel cords generally is regarded as being of particular advantage.
A pipe conveyor of the generic kind in question with overlapping of the conveyor belt at the edges when the belt pipe is formed is known from DE-OS 39 10 911. It consists of rubber or rubber-like plastic and a plurality of steel cords as tension carriers embedded therein and extending in longitudinal direction of the conveyor belt, the tension carriers being disposed so as to be distributed uniformly across the width of the conveyor belt. The steel cords of this known conveyor belt either have a smaller diameter in the region of overlap ("A") than in the remainder of the belt ("B"), or the steel cords have a different strength at those locations. This different design treatment of the conveyor belt structure is intended to meet at least some of the requirements explained above of a tubular conveyor belt. It was found, however, that this is possible only to a limited extent by means of that particular known structure, especially so as regards the curve negotiating characteristic of the conveyor belt with smaller radii of curvature and as regards the length of the trough formation. Furthermore, many times it is more expensive to produce a steel cord conveyor belt with different steel cords or with steel cords which are arranged so as to be distributed differently.